It is known that the muscle cells of the atrial myocardium in mammals contain, in addition to contractile elements similar to those found in ventricular fibers, a highly developed Golgi complex, a relatively high proportion of rough endoplasmic reticulum, and numerous membrane bound storage granules, referred to as specific atrial granules. No such granules appear to exist in ventricular muscle cells. Morphologically and histochemically the atrial granules resemble those present in polypeptide-hormone producing cells (J. Histochem. Cytochem. 26, 1094-1102 (1978) de Bold et al). It is also known (Life Sciences, Vol. 28 pp 89-94 (1981) de Bold et al) that injection of a crude extract of rat atrial myocardium induces a very potent and immediate natriuretic response in non-diuretic assay rats. The problem remains, however, to isolate and identify the atrial natriuretic factor (ANF) contained in the crude extract of the specific atrial granules, and which is hereinafter referred to as cardionatrin I.